This is not uncommon, it is caused by limited download speeds and can be down to many reasons....
Videos continually increase their resolution, and don't care that people's download speeds don't increase as quickly.
Your ISP may throttle your download speed at certain times of day because so many other users are doing immense amounts of downloading. And not just on your local exchange, but down the national pipelines too.
If you have a wireless network, make sure a neighbour is not 'borrowing' some of your download capacity.
A windows update can change your system properties and you have little chance of finding it, MS may actually replace your network driver without asking you. But by all means reinstall the latest network driver you can find. Also check out the properties of your network to make sure as many available settings are giving you the optimal performance. I know this can be difficult to determine, particularly as there is no general advice available.